In certain processes relating to the sorting of particulate material e.g. one sorting, it is necessary to obtain a measurement of a certain property e.g. radioactivity on a per unit volume or mass basis for each particle. This creates the need for some means of rapidly obtaining a measure of the volume of rapidly moving individual particles.
It is known to pass the particles through a light gate including a plurality of closely spaced light beams which are located in a common plane and are focused on to light detectors. The number of beams which are broken by a particular particle in whatever time it takes the particle to traverse the screen enables a computer connected to the screen to determine a two dimensional shadow area of the particle. The area is then multiplied by an empirically determined factor to arrive at the theoretical volume and mass of the particle.
A problem which often leads to substantial radiation measurement errors with this sorting method is that no accurate compensation is made for the three dimensional shape of the particle. For example, the radioactive self absorption and radiation shadow of a cubic particle as seen by a planar radiation detector is greatly different from that of a relatively flat particle containing the same amount of radioactive material and having roughly the same physical two dimensional shadow area as the cubic particle and may well lead to rejection of the cubic particle even though it is of acceptable grade.